1. Field of the Invention
The present invention relates to the field of detection apparatus used to screen for the presence of explosives and other chemical entities.
2. Background Information
Safeguarding the public against illicit chemical attacks is a great concern. Explosives and chemical weapons are two classes of chemicals that can be immediately fatal. Biological weapons involving infectious organisms are also a great concern. It is imperative that new detection technology be capable of detecting an expanding list of chemical threats. It is also desirable to provide a detection system that performs quickly and with high accuracy in order to minimize disruption to the general public due to intolerable waits and excessive false detections.
A mass spectrometer (MS) and an ion mobility spectrometer (IMS) are typically used to detect one or more trace molecules from a sample. For example, a MS and IMS spectrometer can be used to detect the existence of dangerous compounds such as explosives and chemical weapons. MS and IMS detect compounds by ionizing the molecules and measuring their properties under the influence of an electric field.
The detection of explosive compounds by MS and IMS is almost always done by negative ionization due to the high electron affinity of explosives compounds due to their common presence of nitro groups. Other classes of compounds such as chemical weapons and drugs are best detected by positive ionization. Therefore it is desirable to provide a detector that can create both positive and negative ionization.
U.S. Pat. No. 4,849,628 issued to McLuckey et al. discloses an ionizer commonly referred to as a glow discharge ionizer (GDI). GDIs are capable of achieving both positive and negative ionization. The ionizer operates at about 1 torr of pressure. One advantage of a GDI is that at low pressure ion suppression due to reactions of the ions with other trace molecules is minimized. At higher pressure it is possible for the desired ion to react or to lose its charge in collisions with other molecules. However, the GDI source operates with an electric field to maintain the discharge and this causes an acceleration of the ions, which can fragment due to collisions with background gas. This fragmentation is often undesirable. Negative ionization usually occurs only for molecules with high electron affinity, however, positive ionization occurs for most molecules including the background gas, which is typically air.
It is desirable to have a positive and negative ionization source that does not suffer from ion suppression, exhibits minimum fragmentation, and that is specific to trace compounds such as explosives, chemical weapons, drugs and other classes of compounds.
U.S. Pat. No. 5,808,299 issued to Syage discloses a mass spectrometer that contains a photo-ionizer. The photo-ionizer includes a light source that can emit a light beam into a gas sample. The light beam has an energy level that will ionize constituent molecules without creating an undesirable amount of fragmentation. Additionally, the light beam does not ionize common background molecules such as the constituents of air. The molecules are typically ionized at sub atmospheric pressures, which minimizes ion suppression. U.S. Pat. No. 6,211,516 issued to Syage et al. discloses a photo-ionizer for mass spectrometry (MS) that operates at higher pressures including atmospheric pressure. U.S. Pat. No. 6,434,765 issued to Robb et al. discloses an atmospheric pressure photo-ionizer that uses dopant molecules to facilitate the ionization process in a process that involves solvent molecules. The use of dopants or reagent gases to enhance the sensitivity of photo-ionization has been disclosed for ion mobility spectrometry (IMS) in U.S. Pat. No. 5,338,931 issued to Spangler et al. and in U.S. Pat. No. 5,968,837 issued to Doering et al.
Generally photo-ionization produces a positively charged ion. This occurs because the absorption of a photon by a molecule can lead to dissociation of an electron. The Doering patent discloses a method for enhancing formation of negative ions by photo-ionization for IMS by using a high abundance of reagent or dopant molecules. The dopant molecules are chosen to be photo-ionizable. This creates a large abundance of positive photons and electrons. The electrons can then attach to other molecules to form a negatively charged ion.
Conventional methods of forming negative ions include atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). These two methods require a high electric field to operate. The APCI process generates a plasma of positive and negative ions and electrons. Electron attachment and other ion molecule reactions can occur to form desired negative ions. In an ESI process, charged droplets are produced that can either be positively or negatively charged depending on the polarity of the voltage applied to the device. APCI and ESI operate at atmospheric pressure and thus ions that are formed can be suppressed by the abundance of ion-molecule collisions.
It is generally desirable to produce ions, such as negative ions, without having to introduce a supply of dopant molecules. It is also generally desirable to produce ions without the use of electric fields, which can cause undesirable ion molecule reactions.
It is also generally desirable to be able to produce negative ions over a wide range of pressures including atmospheric pressure and higher, and sub-atmospheric pressures.